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Old 11-17-13, 04:49 PM   #1
Servicetech
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Lightbulb Cycling Losses for Gas Furnaces

How much does cycling losses of a gas furnace reduce AFUE vs. Thermal Efficiency? Loss of efficiency for oversized AC units is well documented, but I haven't been able to find anything conclusive on gas furnaces.

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Old 11-17-13, 07:42 PM   #2
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I think it depends on many factors, but you can reduce them by wiring in a circuit to start the fan really slow as soon as the burners are lit. And, of course, any good thermostat will let you slow down the response time.
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Old 11-17-13, 11:51 PM   #3
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The losses seem to be mostly from the time it takes for the heat exchanger to warm up and the losses from heat lost into the ductwork in places that its located where you don't want the heat to go.

For my house, I have all rigid metal ductwork and it takes 10 minutes of burning time before the delta T settles on its way out of a supply register across the house from the furnace. I don't occupy the basement in my house, it is storage space and laundry for me so I don't intentionally need it any warmer than it needs to be. If the ductwork was in an attic or in an unconditioned crawlspace, things get worse since the heat escaping that ductwork is lost to the outside. Of course if it takes 10 minutes to settle on the delta T, I should probably be a little upset that my thermostat kicks the show off at 10 mins of burner time even with the max temp span setting. With the expensive Honeywell units, I'd expect the duct losses to multiply, they have setting in those to cycle multiple times per hour. Mine never cycles twice per hour(as in every 30 minutes or less), no matter what load, even with design load yet the Honeywell's are set to something like 5 or 6 cycles for a condensing furnace. Not sure how that makes sense.

Manual J says that in my situation with supply air above 120f, winter design below 15f, with an enclosed unvented basement that duct loss is 25%.
This feels a little exaggerated to me but every time the system fires up it needs to warm up the ductwork before the full BTU output that is possible(subtract losses with what is still getting lost with hot ducts) gets to the living space, and once the furnace is off any extra heat in those ducts will go to where those ducts are. I think there is more cycling loss through the ductwork than inside a condensing furnace in most cases unless the cycle times are long and the ductwork is insulated.
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Old 11-18-13, 01:00 AM   #4
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One quick test is to tape a shoebox over the thermostat. If the short cycling stops, the cause is some of the supply air hitting the sensor and causing unwanted feedback. Then you either adjust the vents or install a thermostat cover as a permanent fix.
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Old 11-18-13, 04:27 PM   #5
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How much does cycling losses of a gas furnace reduce... Thermal Efficiency?
One way you could approach this would be to compare modulating gas furnace efficiency to non-modulating gas furnace efficiency, since the modulating feature is designed to overcome the cycling losses.

You'd need to be careful in selecting similar units for your comparison. For instance, a lot of the modulating units are also condensing units, which also adds to the efficiency.

I know that with regards to mini-splits, the modulating units (AKA: "inverter technology") made a significant jump in efficiency.

You might also look into modulating gas water heaters and also modulating gas hydronic boilers.

I'd be very interested to see what you find out.

-AC
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Old 11-18-13, 09:08 PM   #6
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I'm not sure that modulating furnaces solve the problem. At lower burn rates, the heat exchanger is colder and will exchange slightly less heat, a condensing furnace will still be over 90% efficient so the loss isn't that great though. Another point is that any losses to the ductwork have now become constant ductwork losses instead of intermittent ones. If going with a modulating furnace, I'd make it a point to use a variable speed or X13 motor and see if you can set it to have a very low temperature rise, if possible, to minimize ductwork losses. The trouble is that you'd need to be sure that you downsize your furnace as much as possible to do this and make sure you have enough ductwork to make sure that you'll get the airflow without raising power use or increasing static pressure too much. I personally wouldn't do a two-stage or modulating furnace unless I could be sure that my temp rise is something like 40-50 degrees or so. That takes a pile of air though 578 CFM for a 25000 BTUhr output (low stage of a two stage Bryant 40k furnace) for a 40 degree temp rise. The lowest air flow setting on that particular furnace is in the 550ish CFM so it seems they've got it right. I'd imagine the X13 type electrically efficient ECM motor like that uses about 200 watts with that kind of airflow but that's just a guess based on my PSC blower that gives me about 620cfm from 288 watts, which isn't so good for me because my 57000BTUhr output puts that temp rise at 85 degrees, so I up the furnace speed so its consuming 324 watts and a slightly more respectable temp rise.

Does anyone have a good idea on how much power an X13 or variable speed ECM blower uses? Does anyone have one of these furnaces where they have measured this or has a link to how much power they use at a specific CFM? I have yet to come across this data yet. Reducing cycling losses by raising CFM but staying within the manufacturer temp rise spec will help reduce losses through ductwork and you'd get more use out of your fuel at the expense of extra electricity. I don't think it would make sense to do this with a PSC motor and definitely not with a shaded pole furnace motor like the one I used to have. I'm actually thinking of upping its speed to the max in January (408 watts) which is about 800 CFM to bring the temp rise to about 65 degrees which puts towards the lower end of the 55-85 temp rise range for my older furnace. I think I'd get pretty close to 55 if I swapped my pleated filter out for a cheap fiberglass one for the airflow but I'd rather do what I can to not clog my A-coil.
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Old 11-19-13, 12:14 PM   #7
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Originally Posted by MN Renovator View Post
I'm not sure that modulating furnaces solve the problem. At lower burn rates, the heat exchanger is colder and will exchange slightly less heat, a condensing furnace will still be over 90% efficient so the loss isn't that great though. Another point is that any losses to the ductwork have now become constant ductwork losses instead of intermittent ones. If going with a modulating furnace, I'd make it a point to use a variable speed or X13 motor and see if you can set it to have a very low temperature rise, if possible, to minimize ductwork losses. The trouble is that you'd need to be sure that you downsize your furnace as much as possible to do this and make sure you have enough ductwork to make sure that you'll get the airflow without raising power use or increasing static pressure too much. I personally wouldn't do a two-stage or modulating furnace unless I could be sure that my temp rise is something like 40-50 degrees or so. That takes a pile of air though 578 CFM for a 25000 BTUhr output (low stage of a two stage Bryant 40k furnace) for a 40 degree temp rise. The lowest air flow setting on that particular furnace is in the 550ish CFM so it seems they've got it right. I'd imagine the X13 type electrically efficient ECM motor like that uses about 200 watts with that kind of airflow but that's just a guess based on my PSC blower that gives me about 620cfm from 288 watts, which isn't so good for me because my 57000BTUhr output puts that temp rise at 85 degrees, so I up the furnace speed so its consuming 324 watts and a slightly more respectable temp rise.

Does anyone have a good idea on how much power an X13 or variable speed ECM blower uses? Does anyone have one of these furnaces where they have measured this or has a link to how much power they use at a specific CFM? I have yet to come across this data yet. Reducing cycling losses by raising CFM but staying within the manufacturer temp rise spec will help reduce losses through ductwork and you'd get more use out of your fuel at the expense of extra electricity. I don't think it would make sense to do this with a PSC motor and definitely not with a shaded pole furnace motor like the one I used to have. I'm actually thinking of upping its speed to the max in January (408 watts) which is about 800 CFM to bring the temp rise to about 65 degrees which puts towards the lower end of the 55-85 temp rise range for my older furnace. I think I'd get pretty close to 55 if I swapped my pleated filter out for a cheap fiberglass one for the airflow but I'd rather do what I can to not clog my A-coil.
Thank you for this useful post and detailed post.

It had made me more certain than ever that I am on the right track, and that I am truly doing the work of god, in disassembling and removing my gas furnace, which I haven't used in four years.

I'll no longer bang my head against the sheet metal elbow duct that stuck out from the wall at the bottom of the basement steps.

I'll no longer have floor ducts that have eaten bits of corn chips, gallons of dust, and escaping screws.

I'll no longer need to engage in territorial war against the squirrels over who actually owns the chimney that has gone cold... they can have it now.

I'll finally be able to rip out all of those horrible ducts and be able to properly insulate under the living room and kitchen floors.

The removal of the furnace will enlarge the space I need to stage work on installing radiant floors in my house.

Thank you.

-AC
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Old 11-19-13, 08:06 PM   #8
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What AC_hacker said.

Radiant floors do resolve ductwork loss problems, remove higher wattage blower fans from the equation, replacing them with what can be(although not always) super low wattage pumps, and you can pull cycles long if the water heating source isn't massively oversized.

The issue is usually that radiant floor systems involve tons of labor to retrofit and for people who aren't doing the retrofit as a DIY project, it would make sense to just spend that money on super-insulating the house. I've been doing the math on this now that my local PUC might be approving my gas company to raise the monthly service fee another $7/month and I'm looking at a positive cost benefit and reduced energy usage by going with mini-split heating. It'll take roughly R40 walls(XPS or polyiso outsulation), R60+ ceiling, and my rotting wood windows(replaced with u-0.2 or better) and siding(also rotting) would be part of the work to get my -13f design load under 10k BTUhr.
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Old 11-20-13, 02:11 PM   #9
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Ok, so I have a question considering a different, more common situation. I Know that many veteran ecorenovators have been there and done this, but lots of others have not. I see this a lot more than I should when working on peoples' homes.

Consider a younger, less mechanically knowledgeable citizen who has just purchased a "pre-owned" suburban or urban home. The layout is what they were looking for, within their budget, and passed inspection. It is a "postage stamp" property of less than 1/3 acre in an established neighborhood. The home is in a decent location relative to the owner's lifestyle. It's "where they want to be" for this phase of life.

After moving in and receiving some utility bills, new homeowner realizes the house is not as energy efficient as it could be. They call the local utility company and schedule an energy audit to find out where their home is on the energy-efficiency scale. What they find is confusing as heck to them.

It is found during the audit that the home is "not too shabby for the neighborhood". The envelope has been sealed up to "acceptable" levels and has been insulated to "above average" value. Not quite energy star, but better than most "affordable" site-built homes are being constructed today in the region. In these sections, the home gets a "B" from the utility.

The heating and cooling system doesn't fare so well in the audit. The 80+ year old ductwork was not replaced when the HVAC system was upgraded from a fuel oil or coal burning furnace/boiler in the early 80's. As usual, the upgraded unit was upsized to push enough airflow for the ancient ductwork. From the whistling registers and vents, it is obvious that the air is just flying through the system. The ducts have been gobbed up with mastic to seal up the leaks, but not insulated. Between the high pressure drop and the exposed ductwork, the auditor estimates that duct losses are 40% or more. The HVAC gets a "D".

During the conclusion of the audit, the homeowner sits down at the kitchen table and is given what seems to be a really expensive sales pitch. The auditor recommends to just gut the whole HVAC system and put in something new. A quick tour of the new systems on the market is given, with "good, better, best" systems of increasing estimated cost, naming major components but not much detail. A "top ten" list of local contractors is left with the report, and the auditor makes a hasty exit.

Now that the homeowner has been given all this information, they figure out they can't afford to do it all at once without experiencing financial hardship. The existing system still works properly, but there is much to be gained as far as energy efficiency. Assume a 10 SEER, 2 1/2 ton AC unit and a 50kbtu, 70% natural gas furnace.

What would you guys recommend as a starting point, assuming the homeowner can swing a couple thousand dollars or so today if it will reduce their utility bill?

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Old 11-20-13, 02:18 PM   #10
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I would think that if their house is that tight and insulation is that good, a 70% efficient furnace isn't going to make bills so high that spending thousands would be worth while. I'd probably ride it out with the current furnace and try to fix the ducting. This can be done relatively inexpensively if you DIY.

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